1. Field of the Invention
The present invention relates to a multilayer printed circuit board and to a method of fabricating the multilayer printed circuit board. More particularly, the present invention relates to a multilayer printed circuit board where a plurality of wires in different wiring layers are electrically connected by via-holes and to a method of fabricating this multilayer printed circuit board.
2. Background of the Invention
As electronic equipment becomes smaller and more compact, the printed circuit board on which such electronic equipment is mounted must have a larger number of wires in a limited space. This requirement can be achieved by increasing the number of circuit layers (also called wiring layers herein) of the printed circuit board and making the board into a multilayer circuit board. A plurality of such circuit layers are then connected to each other through via holes.
A now conventional process for fabricating a multilayer printed circuit board is described with reference to FIG. 1. As shown in FIG. 1, a layer of electrically conductive material 11 is first formed on an electrically insulating substrate 10 (FIG. 1(a)). Then, a layer of photoresist 12 is blanket-coated onto the electrically conductive layer 11 and is selectively exposed to actinic radiation and developed (FIG. 1(b)). The electrically conductive layer 11 is selectively etched using the patterned photoresist layer 12 as an etch mask to form a desired lower wiring layer 13 (FIG. 1(c)) and then the patterned photoresist layer 12 is stripped (FIG. 1(d)). Next, a photosensitive dielectric layer 14 is formed on the lower wiring layer 13 (FIG. 1(e)). This dielectric layer 14 is selectively exposed to actinic radiation and then developed to form one or more photo-via holes 15, thereby uncovering selected areas of the lower wiring 13 (FIG. 1(f)). Thereafter, using any of a variety of metal deposition techniques such as electroless plating or sputtering, an upper wiring layer 16 is formed on the dielectric layer 14 and metal is deposited onto the sidewalls and bottoms of the photo-via holes 15 (FIG. 1(g)). By repeatedly performing these steps, a plurality of electrically connected wiring layers is readily formed.
A multilayer printed circuit board formed as described above is connected to an external circuit through a metal wire such as a gold wire. This is achieved by bonding the gold wire to the board. FIG. 2 is a diagram showing a gold wire bonded to the printed circuit board. As shown in FIG. 2(a), a wire 16 has been conventionally connected to a predetermined connection area on an upper wiring layer 15 formed on a photosensitive dielectric layer 14. In normal wire bonding, the wire 16 is pressed against the connection area using a force of about 50 g to 150 g, while ultrasonic vibration is applied to the connection area so that the gold wire is fixed to the connection area by thermocompression bonding.
One type of photosensitive dielectric material is, for example, epoxy resin having photosensitivity. One such photosensitive epoxy resin is sold (in what is initially liquid form) under the trade name PROBIMER-52 by Ciba Geigy of Japan. Although there are other such materials, this photosensitive resin is susceptible to deformation by high temperature or high pressure. Therefore, when bonding the wire 16 to the bonding pad 15 overlying the photosensitive resin layer 14, there are some cases where the photosensitive resin layer 14 is softened by thermocompression bonding through application of heat and application of pressure and therefore a depressed portion occurs in the direction where the wire 16 is pressed, as shown by reference character A in FIG. 2(a). In addition to the softening caused by heat or pressure, there is the possibility that the size of the depressed portion will be increased by the ultrasonic vibration applied to the bonding pad 15, when bonding is performed.
If wire bonding is performed under the above-described circumstances, there is the possibility that, at the portion B clamped by the capillary head 17 of a bonding machine at the edge of the depressed portion A, the wire 16 will be excessively locally stressed and cut, as shown in FIG. 2(b). On the other hand, if the temperature, applied when bonding is performed, is set to a low temperature in order to prevent the formation of such a depressed portion of the photosensitive resin layer 14, then there will be the problem that a sufficient connection strength cannot be obtained between the wire and the pad and therefore a reduction in reliability of the connection will be caused.